1. Field of the Invention
The present invention is directed to a transmission method for a magnetic resonance signal, wherein the magnetic resonance signal is supplied to an analog-to-digital converter and is digitized thereby, as well as to a corresponding coil arrangement for acquiring the magnetic resonance signal, a corresponding acquisition circuit for digitizing the magnetic resonance signal, and a magnetic resonance signal transmission device.
2. Description of the Prior Art
Magnetic resonance signals exhibit a large range of signal dynamics that partially lies above 90 dB. In order to be able to process such signals without noticeable degradation of the signal-to-noise ratio, the components with which the signal is processed must be able to process an even larger range of dynamics. This usually requires highly linear signal processing electronics with a relatively high power consumption, and also usually requires the employment of two-range analog-to-digital converters. Such components, some of which are special components, are comparatively expensive.
An object of the present invention is to provide a transmission method and corresponding devices with which more economical components can be employed for the transmission of signals with a large dynamic range.
The object is achieved in accordance with the invention in a transmission method wherein the magnetic resonance signal is compressed before being supplied to an analog-to-digital converter, so that high signal values are disproportionately reduced, and the magnetic resonance signal is expanded following the analog-to-digital converter after the digitization, so that high signal values are over-proportionally amplified.
The object also is achieved in accordance with the invention in a coil arrangement having a compressor, with which high signal values can be over-proportionally reduced, arranged between a resonator for the acquisition of a magnetic resonance signal and an in-feed element for feeding the magnetic resonance signal into a transmission path.
The above object is also achieved in a acquisition circuit, wherein a digital-to-analog converter is followed by an expander with which high signal values are over-proportionally amplified.
The inventive magnetic resonance signal transmission device is composed of a resonator for acquiring a magnetic resonance signal, a compressor following the resonator for over-proportional reduction of high signal values, an analog-to-digital converter following the compressor, and an expander following the analog-to-digital converter for over-proportional amplification of high signal values.
The inventive solution causes a high sensitivity for small signal to be achieved at the expense of a somewhat poorer amplitude resolution for large signals. Magnetic resonance signals, however, are already very small during the majority of the reception time span. The brief-duration, high signal maximum lasts for only a short time. A reduced resolution also can be accepted in this range.
Due to the reduced dynamics demands, it is possible in accordance with the invention to optically supply the compressed magnetic resonance signal to the analog-to-digital converter, for example via a light waveguide. It is also possible to supply the compressed magnetic resonance signal to the analog-to-digital converter without any transmission line. In particular, a radio link can be used.
It has proven optimum when the magnetic resonance signal is approximately compressed according to a substantially linear logarithm characteristic. Even when the magnetic resonance signal is compressed roughly according to an exponent characteristic with an exponent factor, however, acceptable results are obtained. When, in particular, the exponent factor is between 0.2 and 0.5, particularly between 0.3 and 0.35, for example at one-third, a compression is achieved with the exponent characteristic that is nearly as good as that deriving with a linear logarithm characteristic.
In an embodiment wherein the expanding ensues by means of a look-up table, the expansion function can be adapted in a simple way to the conditions of the system.
In a further embodiment a number of oscillations that have a uniform oscillatory frequency but different oscillatory amplitudes are supplied to the compressor for determining an expansion function of the expander, and corresponding output amplitudes of the analog-to-digital converter are acquired and the expansion function is determined on the basis of the oscillatory amplitudes and the output amplitudes. The expansion function of the expander thus is determined in an especially reliable way.
In the simplest case, the expanded magnetic resonance signal is proportional to the uncompressed magnetic resonance signal.
The magnetic resonance signal is band-filtered after the expansion. When the signal transmitted via the transmission path is band-limited, it is better when the band-filtered magnetic resonance signal is proportional to the uncompressed magnetic resonance signal.